Polydipsic hyponatremic schizophrenics and 3 control groups will receive an infusion of the psychostimulant methylphenidate to assess if previously defined alterations in vasopressin secretion are linked to psychosis. We predict the relationship between change in psychotic symptoms and vasopressin secretion will be shifted to the left in polydipsic hyponatremic schizophrenics relative to other groups.